Direct sequence spread spectrum (DSSS) is a modulation technique for digital signal transmission over the airwaves. The DSSS phase-modulates a sine wave pseudo-randomly with a continuous string of pseudorandom noise (PRN) code symbols call “chips”, each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. The DSSS uses a signal structure in which the sequence of chips produced by a transmitter is known a priori by a receiver. The receiver may then use the same PRN code sequence to counteract the effect of the PRN code sequence on the received signal in order to reconstruct the information signal.
DSSS transmissions multiply the data or information being transmitted such as, for example, messages from a global navigation satellite system (GNSS) by a PRN code sequence thereby spreading the energy of the original signal into much wider band. The resulting signal resembles white noise and this noise-like signal may be used to exactly reconstruct the original data or information at a receiver such as, for example, a GNSS receiver by multiplying the noise-like signal by the same PRN code sequence. This process, known as de-spreading, mathematically constitutes a correlation of the transmitted PRN code sequence with the PRN code sequence that the receiver believes the transmitter is using. Correlating all possible delays of the transmitted PRN code with the reference PRN code sequence mathematically constitutes convolution. The processing required for convolution is reduced by transforming the PRN codes to the frequency domain.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.